#include <iostream>
#include <vector>
using namespace std;

class Solution {
public:
    int waysToStep(int n) {
        if (n == 1) return 1;
        else if (n == 2) return 2;
        else if (n == 3) return 4;
        vector<int> dp(n + 1);
        dp[1] = 1; dp[2] = 2; dp[3] = 4;
        for (int i = 4; i <= n; ++i)
            dp[i] = ((dp[i - 1] + dp[i - 2]) % 1000000007 + dp[i - 3]) % 1000000007;
        return dp[n];
    }
};

int main()
{
	return 0;
}